Field of Invention
The present invention relates to an image reading apparatus and an image forming system.
Description of Related Art
Heretofore, electrophotographic image forming apparatuses are known as multi-function printer (MFP) which incorporates the functionality of multiple devices such as printers, copying machines and so forth. This type of image forming apparatus performs calibration to adjust characteristics of the image forming apparatus for the purpose of realizing optimal color reproducibility. The calibration is a process of updating the gradation characteristics including densities and the like of respective colors, i.e., C (cyan), M (magenta), Y (yellow) and K (black).
When performing calibration, a plurality of patches having different densities are used. The patch is an image which is formed on a sheet for use in measuring and correcting colors and optically read by a density detecting sensor.
Also, the data obtained by optically reading the patch is compared with the image data which is used for forming the patch, and if there are differences therebetween, printing densities or the like are corrected. The quality of the output image is improved in this manner. It is believed that the size of such a patch is preferably as small as possible in the conveying direction of a sheet for the purpose of reducing a sheet for use in calibration.
The timing of acquiring such a patch is determined by detecting the leading edge of a sheet with a sheet detection sensor, and determining the position of the patch at a predetermined timer interval, and thereby the leading edge of the sheet is used as a reference position. When the leading edge of the sheet is used as a reference position in this manner, the printing position of a patch is affected by the start position of printing and magnification error in the conveying direction of the sheet, and therefore the patch may not be detected in the accurate printing position. The size of the patch is expanded for avoiding such a situation.
Also, in some cases, a positioning image is printed near the leading end of a sheet in the conveying direction; a misalignment confirming image is printed near the tail end of the sheet in the conveying direction; and a patch is printed between the positioning image and the misalignment confirming image. The density detecting sensor reads a patch with reference to the positioning image, and compensates conveyance error with reference to the misalignment confirming image (for example, refer to Japanese Patent Published Application No. 2003-131466).
In accordance with the technique described in Japanese Patent Published Application No. 2003-131466, since the reference position of a sheet is detected without using a sheet detection sensor, there is no error caused by the installation alignment of the sheet detection sensor. Also, in accordance with the technique described in this publication, since conveyance error is compensated, the detection position is prevented from being displaced with time.
However, in accordance with the technique described in this publication, a patch cannot be detected in a correct position if the printing magnification varies. Furthermore, in accordance with the technique described in this publication, since a long patch array is needed in the conveying direction of a sheet, the sheet required for adjusting an image cannot be saved.
In other words, in accordance with the technique described in this publication, it is impossible to save sheets required for adjusting an image while reducing the influence of magnification error and detecting patches in correct positions.
The present invention is made in order to solve such a prior art problem and used to provide an image reading apparatus and an image forming system which can reduce the influence of magnification error, detect patches in correct positions, and save the sheets required for adjusting an image.